Optical transmission systems are generally based on one of two methods of modulation of an optical signal, either direct modulation or external modulation. In the first of these methods, the bias current applied to a laser is modulated, turning the laser “on” and “off”. The disadvantage of this method is that when higher switching speeds are required, the dynamic behavior of the semiconductor material of the laser itself introduces distortion, primarily in the form of chirp. External modulation of an optical signal produces a modulated optical output signal with significantly reduced chirp, and external modulators have become preferred for high speed applications. In particular, electro-optic modulators such as Mach Zehnder interferometers (MZIs) are typically used for high speed applications.
For many years, external modulators have been made out of electro-optic material, such as lithium niobate. Optical waveguides are formed within the electro-optic material, with metal contact regions disposed on the surface of each waveguide arm. The application of a voltage to a metal contact will modify the refractive index of the waveguide region underneath the contact, thus changing the speed of propagation along the waveguide. By applying the voltage(s) that produce a π phase shift between the two arms, a nonlinear (digital) Mach-Zehnder modulator is formed. In particular, the optical signal is launched into the waveguide and the I/O electrical digital signal is applied to the contacts (using proper voltage levels, as mentioned above). The optical output is then “modulated” to create an optical I/O output signal. A similar result is possible with a linear (analog) optical output signal.
Although this type of external modulator has proven extremely useful, there is an increasing desire to form various optical components, subsystems and systems on silicon-based platforms. It is further desirable to integrate the various electronic components associated with such systems (for example, the input electrical data drive circuit for an electro-optic modulator) with the optical components on the same silicon substrate. Clearly, the use of lithium niobate-based optical devices in such a situation is not an option. Various other conventional electro-optic devices are similarly of a material (such as III-V compounds) that are not directly compatible with a silicon platform.
Recent advances have been made in the capability of forming optical devices, such as the modulator described above, within a silicon platform, based on free carrier modulation. In this configuration, the phase-shifting elements forming the modulator arms take the form of MOS capacitors formed along silicon waveguides. An applied voltage induces an accumulation of charges near the gate dielectric of the capacitor which, in turn, modifies the refractive index profile of the waveguide and ultimately the optical phase of the light passing through the waveguide. See, for example, U.S. Pat. Nos. 6,845,198 and 7,065,301, both assigned to the assignee of this application.
The application of the electrical drive signal to the capacitor structures of the phase-shifting elements has been found to be directly impacted by the DC bias voltage of the data itself. A remaining need in this technology is the ability to adjust the DC bias of the high speed electrical data independent of the peak-to-peak swing of the voltage applied to the modulator.